This invention relates to a steering responsive wheel drive system for a four wheel drive vehicle.
On a four wheel drive vehicle with two steered wheels, the two steered wheels travel a longer distance when the vehicle turns than do the non-steered wheels. If such a vehicle has a drive system where the front and rear axles are connected together with gears and shafts so that drive ratio between front and rear axle is constant, the drag from steered wheels not running fast enough will resist vehicle steering and vehicle pulling ability. To counteract this problem, some vehicles are designed so that the drive is disconnected from the steered wheels when the vehicle is turning. But, this drastically decreases the pulling ability of the vehicle.
In conventional mechanical four wheel drive tractors with Ackerman-type steering mechanisms, in order to assure that the steered wheels assist in propelling the vehicle, it is known to overspeed the front wheels by a constant 5% or 6% above the speed required by the Ackerman steering geometry.
Other vehicles are known which counteract this problem by rotating the steered wheels faster than the non-steered wheels during sharp turns, by automatically stepping a gear when the steering angle reaches a predetermined point. For example, U.S. Pat. No. 4,792,009, issued 20 Dec. 1988, and assigned to Kubota, Ltd., discloses a four wheel drive vehicle with a front wheel change speed mechanism which is controlled to produce a high speed state with the front wheels accelerated when a steering angle of the front wheels exceeds a predetermined angle. U.S. Pat. No. 4,723,623, issued 9 Feb. 1988, and also assigned to Kubota, Ltd., discloses a four wheel drive vehicle with a front wheel drive transmission which, in response to a front wheel steering operation, switches to an accelerating mode in which the average speed of the front wheels is greater than the average speed of the rear wheels. But, these systems produce an abrupt step-wise wheel speed change and can cause considerable tire scuffing and soil displacement when operative.
U.S. Pat. Nos. 5,343,971 and 5,465,806 describe systems with motors on either side of the vehicle wherein the motors are controlled to provide differential steering as a function of a sensed steering angle. But, these systems do not provided front wheel speeds which are greater than the rear wheel speeds.